How do we assign a second material to selected faces in 2.8?
This is a known issue when using multiple materials with a number of older GeForce, Quadro and other end of life graphics cards even though they seem to meet the minimum requirements.


  Blender Known Issue: Drawing error with multi-material object. Faces disappear or showing artifacts with 2 or more material slots applied to same object. When trying to use a second material or more on an object, the faces where it is applied become transparent or in some cases they are distorted as black rectangles that stretch to the origin (this second occurs above all with more complex models than a cube). It happens in Blender 2.81 as well as in Blender 2.82, but in Blender 2.80 there is no such error.


While the bug report lists multiple cards, the list is not exhaustive (I have this issue on my card, the GeForce 9800 GTX+, which is not listed). The core issue is the recentness of the video driver. In the case of NVIDIA GPUs, they have to be able to run NVIDIA drivers greater than v400. The highest driver that I can install on the GeForce 9800 GTX+ is GeForce v342.01.

I can confirm that as listed in the bug report, I can use multiple materials in Blender 2.80 without a problem.